1. Field of the Invention
This invention relates to thermoplastic elastomer compositions prepared using hydrosilylation crosslinking of the elastomer component of the composition. A thermoplastic elastomer is generally defined as a polymer or blend of polymers that can be processed and recycled in the same way as a conventional thermoplastic material, yet has properties and functional performance similar to that of vulcanized rubber at service temperatures. Blends or alloys of plastic and elastomeric rubber have become increasingly important in the production of high performance thermoplastic elastomers, particularly for the replacement of thermoset rubbers in various applications. High performance thermoplastic elastomers in which a highly vulcanized rubbery polymer is intimately dispersed in a thermoplastic matrix are generally known as thermoplastic vulcanizates.
2. Description of the Related Art
Polymer blends which have a combination of both thermoplastic and elastic properties are generally obtained by combining a thermoplastic resin with an elastomeric composition in a way such that the elastomer component is intimately and uniformly dispersed as a discrete particulate phase within a continuous phase of the thermoplastic. Early work with vulcanized rubber components is found in U.S. Pat. No. 3,037,954 which discloses both static vulcanization of the rubber, as well as the technique of dynamic vulcanization wherein a vulcanizable elastomer is dispersed into a molten resinous thermoplastic polymer and the elastomer is cured while continuously mixing and shearing the blend. The resulting composition is a micro-gel dispersion of cured elastomer in an uncured matrix of thermoplastic polymer.
In U.S. Pat. No. 32,028 polymer blends comprising an olefin thermoplastic resin and an olefin copolymer are described, wherein the rubber is dynamically vulcanized to a state of partial cure. The resulting compositions are reprocessible. U.S. Pat. Nos. 4,130,534 and 4,130,535 further disclose thermoplastic vulcanizates comprising butyl rubber and polyolefin resin, and olefin rubber and polyolefin resin, respectively. The compositions are prepared by dynamic vulcanization and the rubber component is cured to the extent that it is essentially insoluble in conventional solvents. A range of crosslinking, or curing, agents for the vulcanization of the rubber are described in the early art, including peroxides, sulfurs, phenolic resins, radiation, and the like.
U.S. Pat. No. 4,803,244 generally discusses the use of multifunctional organosilicon compounds in conjunction with a catalyst as an agent for crosslinking the rubber component of a thermoplastic elastomer by hydrosilylation. Hydrosilylation involves the addition of a silicon hydride across a multiple bond, often with a transition metal catalyst. This patent describes a rhodium catalyzed hydrosilylation of EPDM rubber in a blend with polypropylene to produce thermoplastic elastomers having a gel content of up to 34% (after correction for the plastic phase). This degree of vulcanization was achieved only with a high level of catalyst.
A further discussion of hydrosilylation crosslinking of the rubber in a thermoplastic elastomer composition is found in U.S. Pat. No. 5,672,660. The platinum catalyzed hydrosilylation of EPDM rubber containing 5-vinyl-2-norbornene as a diene monomer is described.